1. Technical Field
The present invention relates to a light emitting element, a light emitting device, a display apparatus, and electronic equipment.
2. Related Art
An organic electroluminescence element (a so-called organic EL element) is a light emitting element which has a structure which interposes at least one light emitting layer (a luminous organic layer) between an anode and a cathode. In such a light emitting element, by applying an electric field between the cathode and the anode, electrons are injected into the light emitting layer from the cathode side and holes are injected from the anode side. As a result, excitons are generated by the electrons and holes being recombined in the light emitting layer, that is, by the carriers being recombined. When the excitons return to a ground state, the energy amount thereof is released as light.
A light emitting layer which releases light in this manner normally includes a light emitting material (a light emitting dopant) and a host material.
Here, the host material is generally included at the highest ratio among the constituent components in the light emitting layer. The host material has a role of supporting a film of the light emitting layer. In addition, the host material has a role of moving excitation energy, which is generated by the recombination of carriers in molecules of the host material when a voltage is applied between electrodes, to a light emitting material and causing the light emitting material to emit light. In contrast, the light emitting material is a compound which has fluorescence or phosphorescence and has a role of substantially emitting light by being excited by receiving excitation energy from the host material.
In an organic EL element with such a configuration, the recombination position of the carriers is determined by a carrier transportation property with which the host material, which is used for the light emitting layer, is provided. In addition, in a case of using a luminescent material as the light emitting material, a large number of host materials such as acene-based compounds such as anthracene and naphthacene exert a great bias on a carrier transportation property of the holes and the electrons. Therefore, the recombination positions of carriers concentrate in the vicinity of the interface of the light emitting layer. As a result, the light emitting material in the vicinity of the interface, that is, only a portion of the light emitting material in the light emitting layer, contributes to the light emission. Thus, there is a problem in that local deterioration is promoted in the light emitting material which is positioned in the vicinity of the interface and deterioration in the brightness of the light emitting layer is accelerated.
With respect to this, in recent years, it has been reported that the efficiency of an organic EL element is increased and the life span thereof is extended by controlling the carrier transportation property in the light emitting layer by forming a light emitting layer which includes an assist dopant material for controlling a carrier transportation property as a constituent material other than the light emitting material and the host material (for example, refer to JP-A-2005-108727).
However, in JP-A-2005-108727, the characteristics of the organic EL element are improved by controlling the relationship between HOMO and LUMO of the host material and the assist dopant material. It is not possible to sufficiently control the carrier transportation property of the holes and electrons simply by controlling the relationship between HOMO and LUMO in this manner. For this reason, it is not possible to set the recombination position of the carriers in the light emitting layer to a favorable position.